PLC and History Programmable Logic Controller (PLC) is a user friendly, microprocessor based, specialized computer that carries out control functions of many types and levels of complexity. They are used to monitor crucial process parameters and adjust process operations accordingly. PLC’s will operate any system that has output devices that go on and off. Theses are known as discrete or digital outputs. It can also operate any system with variable (analog) outputs. On the input side it can be operated by on/off devices or by variable (analog) input devices. Some examples of PLC use is CNC machining, building energy and security control systems, and for some new nontraditional new uses such as home and medical equipment. The first PLC evolved from conventional computers in the late 1960’s and early 1970’s. These were primarily installed in automotive plants and this reduced changeover shut down time from a month to a few days. The PLC keyboard programming also replaced rewiring panels full of wires, relay timers, and other components. In 1972 the introduction of the microprocessor chip increased computer power and reduced computing costs. During the late 1970’s, improvements made the PLC more user friendly and programs were written in higher-level language so more people could understand it. In 1980 more computer power per dollar was available. PLC then came into exponentially increasing use. They were more affordable and therefore became the greatest volume product for some large electronics and computer companies. Between 1970 and 1990, the PLC market grew from 80 million to 1 billion dollars per year. Today the market has grown to 1.4 billion dollars and still growing.